class Solution {
public:
    TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
      if(!root1&&!root2) return NULL;
      TreeNode* root = new TreeNode(0);
      if(root1&&root2) root->val = root1->val + root2->val;
      if(root1&&!root2) return root1;
      if(!root1&&root2) return root2;

      root->left = mergeTrees(root1->left,root2->left);
      root->right = mergeTrees(root1->right,root2->right);

      return root;
    }
};
